<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Lazyload</title>
    <style>
      .drag {
        background-color: skyblue;
        position: absolute;
        line-height: 100px;
        text-align: center;
        width: 100px;
        height: 100px;
      }
    </style>
  </head>
  <body>
    <!-- left和top要写在行内样式里面 -->
    <div class="drag" style="left: 0; top: 0">按住拖动</div>
    <script src="./jquery-3.6.0.min.js"></script>
    <script>
      // 获取DOM元素
      let dragDiv = document.getElementsByClassName('drag')[0]
      // 鼠标按下事件 处理程序
      let putDown = function (event) {
        dragDiv.style.cursor = 'pointer'
        let offsetX = parseInt(dragDiv.style.left) // 获取当前的x轴距离
        let offsetY = parseInt(dragDiv.style.top) // 获取当前的y轴距离
        let innerX = event.clientX - offsetX // 获取鼠标在方块内的x轴距
        let innerY = event.clientY - offsetY // 获取鼠标在方块内的y轴距
        // 按住鼠标时为div添加一个border
        dragDiv.style.borderStyle = 'solid'
        dragDiv.style.borderColor = 'red'
        dragDiv.style.borderWidth = '3px'
        // 鼠标移动的时候不停的修改div的left和top值
        document.onmousemove = function (event) {
          dragDiv.style.left = event.clientX - innerX + 'px'
          dragDiv.style.top = event.clientY - innerY + 'px'
          // 边界判断
          if (parseInt(dragDiv.style.left) <= 0) {
            dragDiv.style.left = '0px'
          }
          if (parseInt(dragDiv.style.top) <= 0) {
            dragDiv.style.top = '0px'
          }
          if (
            parseInt(dragDiv.style.left) >=
            window.innerWidth - parseInt(dragDiv.style.width)
          ) {
            dragDiv.style.left =
              window.innerWidth - parseInt(dragDiv.style.width) + 'px'
          }
          if (
            parseInt(dragDiv.style.top) >=
            window.innerHeight - parseInt(dragDiv.style.height)
          ) {
            dragDiv.style.top =
              window.innerHeight - parseInt(dragDiv.style.height) + 'px'
          }
        }
        // 鼠标抬起时，清除绑定在文档上的mousemove和mouseup事件
        // 否则鼠标抬起后还可以继续拖拽方块
        document.onmouseup = function () {
          document.onmousemove = null
          document.onmouseup = null
          // 清除border
          dragDiv.style.borderStyle = ''
          dragDiv.style.borderColor = ''
          dragDiv.style.borderWidth = ''
        }
      }
      // 绑定鼠标按下事件
      dragDiv.addEventListener('mousedown', putDown, false)
    </script>
  </body>
</html>
